Some molecular defects in sickle cell disease have been well-defined, but treatment is still largely only supportive. Further definition of the abnormalities at the level of the erythrocyte and endothelial cell, identification of which abnormalities are important in production of the vasooclusive effect of the disease, and understanding how these abnormalities arise will permit the design of better therapies. The present application proposes studies in three areas: understanding the molecular mechanisms of increased sickle cell/endothelial cell adhesion under physiological conditions of flow, the effect of perfusion of sickle cells on endothelial cell adhesion under physiological conditions of flow, and the consequences of shear stress exposure for the sickle erythrocyte. In the first area, we hypothesize that binding proteins, particularly large forms of von Willebrand factor (vWF) but possibly other adhesive proteins will be employed to dissect the molecular mechanisms involved. Exciting preliminary data indicate that a GPIb-like receptor may be an important component--and that blocking the GPIb binding domain on vWF with aurin tricarboxylic acid (ATA) greatly reduces adhesion under venular flow conditions. In the second area, we hypothesize that interactions under flow of abnormally adhesive and rigid sickle erythrocytes with endothelium lead to alterations of endothelial cell metabolism and function. These changes may include over damage, but also may give rise to altered levels of synthesis of vasoactive and mitogenic compounds (including prostacyclin, endothelial-derived relaxation factor, endothelin, platelet-derived growth factor, tissue plasminogen activator and plasminogen activator inhibitor-type 1). Increased or decreased secretion of these compounds with subsequent interactions with smooth muscle cells or components of the coagulation system may be important in thrombotic problems, vasoocclusion and smooth muscle cell hyperplasia often seen in sickle cell disease. In the third area, studies will be done with sickle erythrocytes and density subfractions thereof to determine effects of various levels of shear stress produced in a constant shear rate rotational viscometer. Effects to be studied include hemolysis, changes in hemolysis, osmotic fragility, deformability, density distribution, and calcium uptake and release. The underlying hypothesis is that not only are SS RBCs abnormally shear sensitive, but also that shear stress in vivo may produce some of he abnormalities observed in these cells. Effects of deoxygenation will be evaluated. Control studies will be carried out in density-fractionated red cells from patients recovering from various anemic states, from asplenic subjects, and from individuals with sickle cell trait. We have developed, i our laboratory, several unique systems to study all three of the proposed areas of research.